Computed tomography (CT) is a technology that has been extensively developed in recent years in the medical field. A CT scanner is a complex instrument that is capable of measuring density throughout a two-dimensional plane passing through a patient or other object. By performing a CT scan along multiple planes displaced from one another in a direction normal to the planes, information concerning density throughout a three-dimensional portion of a patient's body may be obtained.
One nonmedical application for which medical CT scanners are well suited is the nondestructive testing of objects fabricated from materials having densities approximately equal to the density of the human body. One example of such an object is a rocket nozzle fabricated from a carbon-carbon composite. The production of such a nozzle involves a lengthy process that starts with graphite cloth impregnated with phenolic resin, and that includes one or more graphitization steps in which the nozzle is subject to temperatures of 4500.degree. F. Because of the length of a carbon-carbon production process, it is extremely important that the nozzle be inspected for defects at a number of stages in the process. In the past, such defect inspection has primarily been accomplished by means of tangentially directed X-rays.
In a typical tangent X-ray technique, an X-ray source is located approximately six feet from the nozzle, and a film cassette is positioned adjacent the nozzle behind the area of interest. The nozzle is exposed to X-rays from the source for a time period ranging from 35-90 seconds, and the film is then developed. The defects that are of interest, such as wrinkles, foldbacks and porosity, show up in the film as density variations that are referred to as low-density indications (LDIs). The tangent X-ray technique can detect LDI's only if they extend over a long enough path length to cause a difference in X-ray beam attenuation of 2-4 percent. For this reason, only LDIs that are in line with the tangent X-ray beam will be detected. This results in a number of defects remaining undetected unless a large number of tangent angles are selected. In practice, a compromise between complete inspection and representative inspection is usually selected.
The development of X-ray computed tomography during the past 10 years for medical applications has produced the technological breakthrough required for more effective nondestructive evaluation and inspection of carbon-carbon material. The low energy X-ray CT technology developed for medical application on the human body is directly applicable to carbon cones, and provides quantitative density data in addition to qualitative pictures. In particular, a CT system is capable of providing a direct measure of the bulk density of a nozzle or other object at any point of interest, rather than simply along selected lines or tangents. Furthermore, a CT system, unlike photographic film, provides an extremely wide dynamic range that allows density measurements with a resolution of 0.1%.